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Longshore size grading on a boulder beach

Andrew Green, Andrew Cooper and Leslee Salzmann
Longshore size grading on a boulder beach
Journal of Sedimentary Research (October 2016) 86 (10): 1123-1128


Longshore size sorting on boulder beaches has not previously been reported. In a boulder beach comprising beachrock slabs, we report systematic longshore clast size grading. The boulder beach at Mission Rocks, South Africa is deposited on an elevated (+ 3 m MSL) shore platform and comprises imbricated clasts up to 5 m in the a-axis dimension (9 tonnes). The clasts are derived from adjacent intertidal beachrock and eolianite outcrops and are emplaced during high-magnitude wave events. Four distinct downdrift-fining cells are present. Each is 40-50 m long. The sorting is attributed to post-emplacement clast redistribution in which the smallest clasts are transported most frequently (by lower-magnitude storms) and therefore travel farthest downdrift. The updrift cell boundary is marked by boulders (5 m in length) that exceed the transport threshold for all but the most energetic of storms while the downdrift limit contains clasts 0.3 m in length. This mechanism of longshore size sorting does not rely on variations in longshore wave power as does cell development on sand and gravel beaches. The alongshore sorting via multiple high- (and variable-) magnitude events over a period of time, distinguishes these coarse clast deposits from those of single extreme events (whether storms or tsunamis).

ISSN: 1527-1404
EISSN: 1938-3681
Serial Title: Journal of Sedimentary Research
Serial Volume: 86
Serial Issue: 10
Title: Longshore size grading on a boulder beach
Affiliation: University of KwaZulu-Natal, School of Agricultural, Earth and Environmental Sciences, South Africa
Pages: 1123-1128
Published: 201610
Text Language: English
Publisher: Society for Sedimentary Geology, Tulsa, OK, United States
References: 28
Accession Number: 2016-096736
Categories: Sedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, sketch maps
S28°16'00" - S28°16'00", E32°30'00" - E32°30'00"
Secondary Affiliation: Council for Geoscience, ZAF, South Africa
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by SEPM (Society for Sedimentary Geology), Tulsa, OK, United States
Update Code: 201647
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